
á
PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
223
the Receive DS3 Framer will then transition to the
“In-Frame” state. At this point, the Receive DS3
Framer will declare itself in the “In-Frame” condition,
and will begin “Frame Maintenance” operations. The
Receive DS3 Framer will then indicate that it has tran-
sitioned from the “OOF” condition into the “In-Frame”
condition by doing the following.
Generate a “Change in OOF Condition” interrupt to
the local μP
Negate the RxOOF output pin (e.g., toggle it “l(fā)ow”).
Negate the “Rx OOF” bit-field (Bit 4) within the
Receive DS3 Configuration and Status Register.
The user can configure the Receive DS3 Framer to
operate such that ‘valid parity’ (P-bits) must also be
detected before the Receive DS3 Framer can declare
itself “In Frame”. The user can set this configuration
by writing the appropriate data to the Rx DS3 Config-
uration and Status Register, as depicted below.
The following table relates the contents of this bit field
to the framing acquisition criteria.
Once the Receive DS3 Framer is in the “In-Frame”
condition, normal data recovery and processing of
the DS3 data stream begins. The maximum average
reframing time is less than 1.5 ms.
7.1.2.2.2
When the Receive DS3 Framer is operating in the
“In-Frame” state (per Figure 68), it will then begin to
perform “Frame Maintenance” operations; where it
will continue to verify that the F- and M-bits are
The Framing Maintenance Mode
present, at their proper locations. While the Receive
DS3 Framer is operating in the “Frame Maintenance”
mode, it will declare an “Out-of-Frame” (OOF) condi-
tion if 3 or 6 F-bits (depending upon user selection)
out of 16 consecutive F-bits are in error. The user
makes this selection for the “OOF Declaration” criteria
by writing the appropriate value to bit 1 (F-Sync Algo)
of the Rx DS3 Configuration and Status Register, as
depicted below.
Rx DS3 Configuration and Status Register, (Address = 0Eh)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
Rx AIS
Rx LOS
Rx Idle
Rx OOF
Int LOS
Disable
Framing on
Parity
F-Sync Algo
M-Sync Algo
RO
RO
RO
RO
R/W
R/W
R/W
R/W
T
ABLE
46: T
HE
R
ELATIONSHIP
BETWEEN
THE
CONTENTS
OF
B
IT
2 (F
RAMING
ON
P
ARITY
)
WITHIN
THE
R
X
DS3
C
ONFIGURATION
AND
S
TATUS
R
EGISTER
,
AND
THE
RESULTING
“F
RAMING
A
CQUISITION
C
RITERIA
”
F
RAMING
ON
P
ARITY
(B
IT
2)
F
RAMING
A
CQUISITION
C
RITERIA
0
The “In-frame” is declared after F-bit synchronization (10 F-bit matches) followed by M-bit
synchronization (M-bit matches for 3 DS3 M-frames)
1
The “In-frame” condition is declared after F-bit synchronization, followed by M-bit syn-
chronization, with valid parity over the frames. Also, the occurrence of parity errors in 2
or more out of 5 frames starts a frame search
Rx DS3 Configuration and Status Register, (Address = 0Eh)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
Rx AIS
Rx LOS
Rx Idle
Rx OOF
Int LOS
Disable
Framing on
Parity
F-Sync
Algo
M-Sync
Algo
RO
RO
RO
RO
R/W
R/W
R/W
R/W